Dry cleaning processes using pressurized carbon dioxide (CO.sub.2) are well known in the art. Dry cleaning systems using liquid/supercritical dense-phase gas such as carbon dioxide are described, inter alia, in U.S. Pat. Nos. 5,267,455 and 5,412,958, 5,316,591, 4,012,194, 5,013,366, 5,456,759 and 5,339,844. In such systems, pressurized liquid CO.sub.2 is pumped from a reservoir into a cleaning chamber, where articles to be cleaned, e.g., clothes, are suspended in the liquid CO.sub.2. Agitating of the articles and/or the CO.sub.2 in the cleaning chamber provides the mechanical action required for cleaning. Some prior art systems use a mechanical rotation mechanism to provide the agitation necessary for cleaning. Other prior art systems use a plurality of injection ports to inject high-pressure liquid CO.sub.2 jets into the cleaning chamber and, thereby, to provide the agitation necessary for cleaning.
Liquid CO.sub.2 may be injected into the cleaning chamber via different sets of injection ports to provide agitation and, consequently, rotation of the articles within the cleaning chamber, in either a clockwise or counter-clockwise direction. In a standard CO.sub.2 dry-cleaning process, the articles are alternately rotated in either direction by periodically stopping the injection through a first set of injection ports and resuming injection of the liquid CO.sub.2 through a second set of injection ports that are positioned to inject the liquid CO.sub.2 in a direction opposite that of the first set of ports. During the injection process, the continuous supply of liquid CO.sub.2 forces the liquid CO.sub.2 in the chamber to be continuously displaced out of the cleaning chamber and returned to the storage tank. After a desired number of agitation cycles are completed, the cleaning chamber is drained and the liquid CO.sub.2 is transported back into the storage tank. A heavy-duty positive displacement piston pump is typically used to circulate the liquid CO.sub.2 throughout the system, e.g. to provide a substantially continuous flow of liquid CO.sub.2 through the cleaning chamber during agitation.
The use of such a pump has a number of disadvantages that render prior art systems complex and/or cost-inefficient for many applications. One disadvantage is that the pump is a relatively expensive element of the dry cleaning system. Another disadvantage is that the pump requires a net positive suction head ("NPSH"). This head is generated by both the fluid level in whatever vessel is to be drained and the elevation of the vessel relative to the pump inlet. Configurations that provide adequate pressure such as tall vessels or mounting the vessel about the pump are not desirable because they result in a large machine. Furthermore, completely draining the cleaning chamber still may be difficult because NPSH decreases as the chamber empties.
Another prior art method of providing adequate pump head is by using a distillation chamber. Gas is heated in the chamber, and the resultant pressure increase is used to provide NPSH. However, the use of such distillation chamber adds complexity and cost to the system.
Furthermore, the pump is susceptible to damage and wear from dirt suspended in the fluid, which reduces pumping efficiency. Filters cannot be used on the suction side of the pump because they decrease the pressure at the pump inlet, adding to the problem of attaining adequate positive pressure head. Thus, in addition to equipment and operating costs, frequent maintenance is also necessary.